This invention relates in general to a communications interface device whereby a facsimile transceiver is enabled to send and receive copies of documents over a two-way voice radio transceiver; and more particularly to a communications interface device which contains electrical interface means which, when connected to a standard facsimile transceiver, simulate a two-wire public switched telephone line; which contains facsimile data modulator and demodulator means for converting facsimile image data into audible tones, and audible tones into facsimile image data; which contains destination addressing means to allow a user to selectively transmit a facsimile message to any one of multiple compatible devices on the same channel; which provides error detection and correction means which assure error-free communications over the radio channel; which contains radio interface means which, connected to a voice radio, simulate an external microphone, talk switch, and speaker, and wherein the data transmission speed is automatically optimized for existing radio channel conditions.
Facsimile transceivers have commonly been used to send and receive copies of written documents over two-wire voice telephone lines. Such a facsimile transceiver contains station selection (dialing) means to place calls over the public switched voice telephone network; answering means to detect ringing signals from the telephone central office; and electrical coupling means to send and receive analog electrical signals in the voice frequency range, at signal levels and frequencies compatible with the voice telephone network. Such a facsimile transceiver also contains optoelectronic document scanning means, which, for each horizontal scan line of a predetermined height across the document to be copied, produce a string of digital data, where, for example, a binary one digit represents a black picture element of a predetermined size, and a binary zero digit represents a white picture element. Such a transceiver also contains modulation means which, for transmission, convert digital image data into tones in the voice frequency range, as well as demodulation means which, for reception, convert audio tones into binary image data. Such a facsimile transceiver also contains hard-copy printing means, such as a thermal print mechanism which makes images by selectively heating elements on a thermal print head in contact with thermally-sensitive paper, or a laser print mechanism which produces images on bond paper using a xerographic process, or similar print mechanism; such printing means converting the received digital image data into a printed document, and reproducing a copy of the transmitted document. Such facsimile transceiver also includes a timing and control means which control and coordinate the above elements in accordance with procedures and protocols established by telephone authorities and by international telecommunications standards bodies.
There are many applications where it is desirable to send and receive documents using such a facsimile transceiver, but where telephone lines are not available. Examples include mobile vehicles, temporary work locations, and remote and undeveloped areas. One such radio data system is the facsimile-radio communication system described in related U.S. patent application Ser. No. 685,831, filed Apr. 16, 1991. As set forth in detail in this patent application, the facsimile radio communications interface device enables a facsimile transceiver to send and receive copies of documents over a two-way voice radio transceiver, and includes electrical interface means which, when connected to a standard facsimile transceiver, simulate a two-wire public switched telephone line; facsimile data modulator and demodulator means for converting facsimile image data into audible tones, and audible tones into facsimile image data; destination addressing means to allow a user to selectively transmit a facsimile message to any one of multiple compatible devices on the same radio channel; error detection and correction means which assure error-free communications over the radio channel; and radio interface means which, connected to a voice radio, simulate an external microphone, talk switch, and speaker.
High data transmission speeds are desirable in any system where digital information is transmitted over radio. High transmission speeds are especially desirable when large amounts of data must be communicated, as in facsimile-radio communication systems. Facsimile images are data-intensive, typically requiring as many as 262,000 bits (binary digits) or more to be transmitted to reproduce each 81/2 by 11 inch page.
It would be ideal for a radio data system to be able to transmit all data at the fastest available speed, for example, 9600 bps (bits per second). Unfortunately, in practice higher data speeds may be realizable only when radio propagation conditions are very good. When the received radio signal strength is weak, or when the radio channel is noisy, it may be necessary to use a lower speed.
It is known to those familiar with the art of digital data communications that the BER (bit error rate), or the probability that a given binary digit will be received in error, tends to increase as the noise level on the communications channel increases relative to the strength of the signal. For example, one common method of sending digital data by radio is to encode binary digits in the form of audible tones in the voice frequency spectrum of from 300 to 3400 Hz (Hertz, or cycles per second), and then to transmit these tones over the radio channel. Encoding schemes for sending digital data may include, for example, FSK (frequency shift keying), whereby binary 1s and 0s are sent as audio tones with two distinct frequencies; MSK (minimum shift keying), a variation of FSK in which the frequency shift is minimized to reduce RF channel bandwidth; PSK (phase shift keying), whereby binary digits are encoded as changes in the phase of an audio frequency tone; or QAM (quadrature amplitude modulation), whereby binary digits are encoded using both the phase and the amplitude of an audio frequency tone; and other encoding techniques may also be employed.
The relationship of BER to communications channel noise level may be understood by the example of one commercially available modem integrated circuit, which supports data speeds of 9600, 7200, 4800, 2400, and 300 bps, as follows. For example, assume a desired BER of 10-4 (1/10,000), or an error rate whereby a single bit error is probable for every 10,000 bits. To achieve a BER of 10-4 at 9600 bps for this modem circuit, the SNR (signal to noise ratio, the quotient of the power of the received audio signal tones, representing the transmitted binary digits, divided by the power of the noise component of the received audio signal, caused primarily by noise on the communications channel) must be 21 dB (decibels). By reducing the data speed from 9600 bps, this modem device can maintain the same desired 10-4 BER level in the presence of increasingly unfavorable communications conditions (decreasing SNR), as follows:
At 9600 bps, requires 21 dB SNR, or noise voltage of up to 9% of signal voltage. PA1 At 7200 bps, requires 17 dB SNR, or noise voltage of up to 14% of signal voltage. PA1 At 4800 bps, requires 15 dB SNR, or noise voltage of up to 18% of signal voltage. PA1 At 2400 bps, requires 8 dB SNR, or noise voltage of up to 40% of signal voltage. PA1 At 300 bps, requires 5 dB SNR, or noise voltage of up to 56% of signal voltage. PA1 1. Channel reservation packet. Sent at network packet speed from the message originator to the message receiver. Includes the address of the sending unit, the address of the receiving unit, and a proposed time period (in seconds) for which the channel is to be reserved. This reservation time is calculated by the transmitting device to allow sufficient time for transmission of the data packet it is prepared to send, plus time for the other network packets included in this data packet sequence. PA1 2. Reservation acceptance packet. Sent at network packet speed from the message receiver, back to the message originator. Includes the addresses of both units, and the time period (in seconds) for which the channel will be reserved. PA1 3. Information data packet. Sent at data packet speed from the message originator to the message receiver. PA1 4. Information acknowledgment data packet. Sent at network packet speed from the message receiver, back to the message originator. When sent, positively acknowledges receipt of the information data packet just transmitted.
While these specific figures on noise sensitivity would not apply exactly to other specific modem devices, it is generally the case that by decreasing data speed (in bps), a greater effective immunity to noise can be achieved.
Requiring users to select a single high fixed speed (such as 9600 bps) might limit transmissions to only those situations where very good reception can be expected. On the other hand, requiring users to select a single, relatively lower fixed speed (such as 2400 bps) might needlessly slow down transmission speeds when higher speeds are often possible.
To provide the greatest flexibility, it is desirable to allow devices to communicate at multiple speeds--for example, at 9600, 7200, 4800, 2400, and 300 bps--on an adaptive basis. The devices should automatically use the highest speed that produces acceptable error rates. For example, the criterion might be established that the link should provide a specified modem BER, for example, 10-4. Two units close together with good radio propagation might be able to communicate at 9600 bps. Two more distant stations may have to use a lower speed, for example, to 2400 bps.
To avoid channel congestion and assure reasonable network message throughput in a multi-user radio data network, it is desirable to design the communication system such that packets are received free of errors most of the time. Otherwise, an excessive proportion of the transmission time available on the network will be dedicated to packet retransmissions, using an error detection and correction scheme such as that described for facsimile-radio communication systems in related patent application Ser. No. 685,831. For example, assume that the BER experienced under prevailing radio propagation characteristics is 10-4 ; (1 in 10,000), and that information is transmitted in packets containing 1,024 bits. This BER could be expected to result in a faulty packet about once every 10 packets (10,240 bits). Approximately 10% of the packets would have to be retransmitted. A 10% packet retransmission rate is tolerable, but if packet retransmission increases significantly beyond that ratio, channel throughput would be degraded to the point that users could not expect prompt and reliable communications.
When communicating over the telephone network, facsimile transceivers automatically change their data speed to adapt to noise levels on the telephone lines. They implement a standard procedure according to a protocol established by international communications standards. The facsimile connection starts at the relatively low speed of 300 bps, during which the sending and receiving transceivers exchange data concerning their respective capabilities, including the maximum data speed at which each is capable of communicating. Assuming that both facsimile transceivers have signaled that they can communicate at a 9600 bps rate, the originating party then sends a training sequence (a predetermined data pattern) at 9600 bps; then drops back to 300 bps and listens. The answering party sends back a 300 bps message which either positively acknowledges reception of the training sequence at 9600 bps--in which case both facsimile transceivers switch to 9600 bps and data communications starts--or else negatively acknowledges training--in which case the originator selects the next lower available speed (typically 7200 bps), and sends a training sequence at this lower speed--repeating until training is successful. If training is unsuccessful at all available data speeds, the call cannot be completed.
In a voice telephone system, the switched telephone network provides a separate circuit path for each voice connection, so that simultaneous conversations do not mutually interfere. However, in a radio environment, multiple devices share the same radio channel, to allow connectivity among many users, and to optimize usage of radio frequency spectrum. Because more than two data modem devices may often be active at the same time, thus interspersing packets, the use of multiple data speeds may give rise to transmission errors.
It is desirable in any network of communications devices for each device to continually monitor the communications channel, so as provide orderly access to the channel, and provide suitable responses to all signals sent to that device. In a network of multiple radio data units where transmissions may occur at varying data speeds, traffic on the radio channel may be unintelligible as among units which have selected varying data speeds. Owing to the complexity of the data demodulation processes involved, practical data demodulator circuits can successfully receive only one data speed at a time. The unit must be set to any one of (for example) 300, 2400, 4800, 7200, or 9600 bps, and thereafter can decode only that data speed, until reset. Suppose a network of three radio data communications units: #1, #2, and #3. Units #1 and #2 are "connected" at 4800 bps; that is, a previous exchange of packets between units #1 and #2 has been made, resulting in a virtual "connection" over the radio, at 4800 bps. While this connection is in place, unit #3 attempts to "connect" to unit #2 by sending a 9600 bps connection request packet, inserted into a time gap in packets exchanged between units #1 and #2. In this event, unit #2 should detect the connection request packet from unit #3, and return a busy packet to unit #3 to indicate that it cannot currently make a connection. If no provision were made to accommodate the differing data speeds, then the 9600 bps connection request packet could not readily be detected; no busy packet would be returned; and the operator of unit #3 would have no way to distinguish whether the intended recipient of his message is busy, or the equipment is out of order. This would be undesirable, since any communications network should provide users with indications as to message status to provide confidence in the integrity of the system.
Further, when set to one specific data speed, a demodulator device may not be able to distinguish between valid data at a different speed, and noise on the radio channel, and might therefore attempt to transmit a packet while another unit is transmitting at an incompatible speed, thereby causing packet collisions, and adversely affecting channel throughput.
Despite the desirability of multi-speed data operation for the aforesaid reasons, multiple data speeds have in the past been impractical in radio data networks, since, owing to the practical limitations of modem devices, packets sent at different data speeds would be mutually unintelligible. This would violate the requirement for continual communications status monitoring by all modem devices, and in some cases lead to unnecessary packet collisions. These problems have in the past restricted practical multi-user radio data modem networks to operation at a single fixed speed.
It is therefore a primary object of the present invention to provide a radio data communication system which permits multiple radio data communication devices to operate at differing data speeds over the same radio channel, optimizing data throughput by transmitting at higher speeds when radio propagation conditions permit, and at lower speeds when necessary to overcome poor signal conditions.
A more specific object of the present invention is to provide a means of determining the optimum information packet speed for a pair of radio data communication devices, based on prevailing radio propagation conditions, by causing each transmitting station to send a predetermined data sequence to the receiver, and the receiving station to return a packet indicating whether the data sequence was received with an error rate no greater than a predetermined level; if successful, by establishing that data speed as the information packet speed to be used in subsequent transmissions of information packets, but if the error rate was exceeded, by causing the transmitting station to reduce its speed to the next lower available speed, and to generate a new data sequence--repeating such procedure until the desired error rate is achieved, whereupon this lower data speed is adopted as the information packet speed.
A further object of the present invention is to regulate channel access by radio data communications units which may have adopted varying data speeds, and to avoid collisions that might otherwise be caused due to the mutual unintelligibility of packets sent at varying speeds, by establishing a network packet speed, typically the lowest available data speed; by causing all network control packets to be transmitted at this network packet speed; by causing a transmitting unit to generate a channel reservation packet, at network packet speed, reserving the channel for a specified number of seconds, prior to sending an information packet at a higher information packet speed; by causing a receiving unit to generate a reservation acceptance packet, at network packet speed, to signal its recognition of the upcoming information packet; and by causing all units in the respective vicinities of the transmitter and the receiver to take notice of such reservation packets, so that they will not attempt to gain channel access during the specified channel reservation period, thereby avoiding packet collisions.
Various means have been developed to send and receive digital data over wireless radio links. A radio modem, for example, may be connected between a computer or data terminal and a radio transceiver. The means for data transfer between the computer or data terminal and the radio modem comprise a direct electrical connection of two closely-located data devices, for example, an RS-232 serial data interface (Electrical Industries Association Standard RS-232). The radio modem contains means for converting digital data received from the computer or data terminal into electrical signals which modulate the carrier of the connected transceiver. Conversely, the radio modem demodulates signals received from the radio so as to regenerate digital data, and passes the data to the connected computer or data terminal.